Magnetic resonance imaging (MRI)
Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging
(NMRI), is primarily a medical imaging technique most commonly used in
radiology to visualize the structure and function of the body. It provides detailed
images of the body in any plane.
Features of MRI
1. MRI provides much greater contrast between the different soft tissues of the
body than computed tomography (CT) does, making it especially useful in
neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer)
2. Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to
align the nuclear magnetization of (usually) hydrogen atoms in water in the body.
3. Radiofrequency fields are used to systematically alter the alignment of this
magnetization, causing the hydrogen nuclei to produce a rotating magnetic field
detectable by the scanner. This signal can be manipulated by additional magnetic
fields to build up enough information to construct an image of the body.
The body is mainly composed of water molecules which each contain two
hydrogen nuclei or protons. When a person goes inside the powerful magnetic field
of the scanner these protons align with the direction of the field.
A second radiofrequency electromagnetic field is then briefly turned on causing the
protons to absorb some of its energy. When this field is turned off the protons
release this energy at a radiofrequency which can be detected by the scanner. The
position of protons in the body can be determined by applying additional magnetic
fields during the scan which allows an image of the body to be built up. These are
created by turning gradients coils on and off which creates the knocking sounds
heard during an MR scan.
Diseased tissue, such as tumors, can be detected because the protons in different
tissues return to their equilibrium state at different rates. By changing the
parameters on the scanner this effect is used to create contrast between different
types of body tissue.
Contrast agents may be injected intravenously to enhance the appearance of blood
vessels, tumors or inflammation. Contrast agents may also be directly injected into
a joint, in the case of arthrograms, MR images of joints. Unlike CT scanning MRI
uses no ionizing radiation and is generally a very safe procedure. Patients with
some metal implants, cochlear implants, and cardiac pacemakers are prevented
from having an MRI scan due to effects of the strong magnetic field and powerful
Applications of MRI
In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain
tumor) from normal tissue. One advantage of an MRI scan is that it is harmless to
the patient. It uses strong magnetic fields and non-ionizing radiation in the radio
frequency range. Compare this to CT scans and traditional X-rays which involve
doses of ionizing radiation and may increase the risk of malignancy, especially in a
The typical MRI examination consists of 5-20 sequences, each of which is chosen
to provide a particular type of information about the subject tissues. This
information is then synthesized by the interpreting physician.
Functional MRI or functional Magnetic Resonance Imaging (fMRI) is a type
of specialized MRI scan. It measures the hemodynamic response related to neural
activity in the brain or spinal cord of humans or other animals. It is one of the most
recently developed forms of neuroimaging. Since the early 1990s, fMRI has come
to dominate the brain mapping field due to its low invasiveness, lack of radiation
exposure, and relatively wide availability.
fMRI image of Brain
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